154 cases of breast cancer (30 T1-52; 124 T3-54) treated by irradiation followed by radical mastectomy were subjected to histological revision in order to check the efficacy of radiotherapy in the local control of cancer and to seek correlations between the histological findings and the clinical course of the disease. The tumor disappeared completely only in 24% of cases, the percentage being higher in the initially smaller tumors; the axillary lymph nodes showed disappearance of the metastases in a still smaller proportion of cases, namely 15-20%. Only in 16 cases (10% of cases) were the histological findings negative both at mammary and axillary levels. X-ray therapy and cobalt therapy were of equal value in the local control of the disease, despite the higher doses of cobalt. The patients operated on after radiotherapy had a decidedly better survival rate than those treated with irradiation only, since the former were selected because of their better clinical course and operated on because of this. Of the operated patients those whose axillary histology was negative presented a higher survival rate (75% of patients living) than those whose findings were positive (53%). Factors of negligible importance prognostically were: complete sterilization at mammary and axillary level after radiotherapy, persistence of florid cancer tissue at mammary level and histiocytosis of the axillary lymph nodes. In short, the result of locoregional therapy, even when successful, was not crucial in the general control of the disease, especially in advanced locoregional breast cancer (T3-T4 with axillary metastases).